diffdecode.js

var args = message.getContent().split(" ");
if (args.length == 1) {
    channel.message(args[0] + " <word>");
    return;
}

var result = "";
for (var i = 1; i < args[1].length; i++)
    result += String.fromCharCode(args[1].codePointAt(i) - args[1].codePointAt(i-1) + 65);

channel.message(result);